1. Field of the Invention
The present disclosure relates to a semiconductor integrated circuit, and more particularly, to an amplifier circuit and a method of generating bias voltage in an amplifier circuit.
2. Description of the Related Art
Differential amplifier circuits are widely used in a semiconductor memory device such as dynamic random access memory (DRAM). The differential amplifier circuits amplify only a voltage difference between voltages applied to two input terminals. If the same voltage is applied to two input terminals, the output voltage of the differential amplifier circuits is not affected.
FIG. 1 is a circuit diagram of a conventional differential amplifier circuit 100. Referring to FIG. 1, the differential amplifier circuit 100 includes a bias circuit (or a bias voltage generating circuit) and a differential amplifier. The differential amplifier circuit 100 can be, for example, a data output driver of a semiconductor memory device.
The bias circuit includes a current source 105, an NMOS transistor 110 in a diode configuration, and a bias capacitor CB to prevent bias voltage noise from being generated in a bias node NB.
The NMOS transistor 110 performs as a voltage source. A terminal of the current source 105 is connected to a source voltage VDD, a source of the NMOS transistor 110 is connected to a ground voltage VSS, and a terminal of the bias capacitor CB is connected to the ground voltage VSS.
The differential amplifier includes load resistors R1 and R2 connected to the source voltage VDD, input NMOS transistors 115 and 120 respectively connected to first and second input voltages VIN1 and VIN2, and a bias transistor 125 including a source connected to the ground voltage VSS. The bias transistor 125 is an NMOS transistor and performs as a current source transistor.
The bias circuit provides constant bias voltage to a gate of the bias transistor 125 of the differential amplifier through the bias node NB.
The differential amplifier generates an output voltage VOUT through an output node NO by amplifying a voltage corresponding to the difference between the first and second input voltages VIN1 and VIN2. Levels of the first and second input voltages VIN1 and VIN2, respectively, input to the input NMOS transistors 115 and 120 of the differential amplifier can swing between the source voltage VDD and the ground voltage VSS.
A turn-on operation (or an activation operation) of the differential amplifier circuit 100 will now be described below.
The bias transistor 125 pulls down a voltage of a common node NC to the ground voltage VSS in response to a bias voltage generated in the bias node NB. When the first input voltage VIN1 is activated to a logic high level (for example, source voltage VDD) and the second input voltage VIN2 is at a logic low level (for example, the ground voltage VSS), the first input transistor 115 is turned on and the differential amplifier circuit 100 performs a turn-on operation. Thus, the output voltage VOUT, i.e., the voltage of the output node NO, is at a logic low level. A turn-off operation of the differential amplifier circuit 100 is performed when both of the first and second input voltages VIN1 and VIN2 are at a logic low level (for example, the ground voltage VSS).
Since current is provided to the common node NC through the load resistor R1 when the first input transistor 115 is turned on, a voltage of the common node NC can be higher than the ground voltage VSS. A voltage of the common node NC can increase a voltage of the bias node NB by parasitic coupling capacitance CC between the common node NC and the bias node NB. Noise corresponding to voltage rising of the bias node NB is referred to as “bias kick-back noise” or “kick-back noise.” Since the bias transistor 125 is turned on strongly by the kick-back noise, a high electric current can flow through the bias transistor 125. Accordingly, power consumption of the differential amplifier circuit 100 increases. Also, noise such as a swing range change of the output voltage VOUT can occur due to the kick-back noise affecting voltage of the bias node NB.
FIG. 2 is a graph of bias voltage variation generated in the bias node NB of FIG. 1 versus time. That is, FIG. 2 indicates a voltage variation of the bias node NB of FIG. 1 in the turn-on and turn-off operations of the differential amplifier circuit 100 of FIG. 1.
Referring to FIGS. 1 and 2, at time TON when the first input transistor is turned on, the bias voltage is increased from a level of the constant bias voltage VB1 to a higher voltage than the voltage VB1 by the coupling capacitance. Noise corresponding to voltage rising of the bias voltage indicates the kick-back noise.
The following known approaches have been used to reduce the kick-back noise. The first approach is to increase the amount of current flowing through the NMOS transistor 110 of FIG. 1 by increasing the current amount of the current source 105 of FIG. 1. The second approach is to increase the size of the bias capacitor CB of FIG. 1. The third approach is to provide a counter coupling capacitance, which has opposite polarity to the coupling capacitance generated in the bias node NB of FIG. 1, to the bias node NB. However, the first approach can increase power consumption; the second approach can increase the total area of the differential amplifier circuit 100; and the third approach can also increase the total area of the differential amplifier circuit 100, since an extra circuit is required to generate the counter coupling capacitance provided to the bias node NB.